Printing devices for printing images by forming dots on a printing medium such as an inkjet printer or the like are widely used as output device of image equipment such as computers, digital cameras, and the like. Also, in recent years, various output devices are starting to be used that display images by forming bright spots on a screen such as liquid crystal, organic LED, or the like, and it is thought that these will be widely used in the future.
To display images on computers or digital cameras using these output devices, it is necessary to perform image processing called color conversion processing and halftone processing on the image data. This is because the color image data expression method differs as shown below by the image data creation side (computer or the like) and the image display side based on the image data (printing device or the like).
First, the method for expressing hue differs by the device for performing image processing such as a computer or the like and by the device for performing image output such as a printing device or the like. Specifically, with a computer or the like, in comparison to expressing hues by combining components for color expression of R (red), G (green), and B (blue), with a printing device, hues are expressed using a combination of C (cyan), M (magenta), and Y (yellow), specifically, inks for absorbing other color light. In this way, the hue expression format using the colors RGB is sometimes called the RGB color system. In comparison to this, the hue expression format using the colors CMY as a base is sometimes called the CMY color system. Also, if we look closely even in cases of using the same RGB color system such as with an LED, organic LED, a computer or the like, in almost all cases, there is a subtle difference in the color of the colors RGB. Therefore, even when using the same RGB color system, to correct the difference in colors, it is necessary to convert the color image data. In this way, to express color images such as of a computer or digital camera or the like with the correct hue, it is necessary to convert the color image data expressed by the color system used with the image processing device side such as a computer or the like to the color image data according to the color system used on the side of the printing device or the like. The color conversion process is the process of performing this kind of conversion.
The method of expressing gradations is also different between the image processing device such as a computer or the like and the output device such as a printing device or the like. Specifically, with a computer or the like, pixels, which are the minimum unit of an image, can express many gradations with that individually, for example if there is 1 byte of image data, it is possible to express 256 gradations with each pixel individually. In comparison to this, with a printing device or an image display device, basically, there are only two states of whether or not dots or bright spots are formed, and therefore, pseudo expression of gradations done by changing the formation density of the dots or bright spots is the method that is mainly used. From this fact, a process is needed for converting image data that can express many gradations for each pixel (gradation image data) to image data that is pseudo expressed in gradations (pseudo gradation image data). Halftone processing is a process for performing this kind of conversion.
Various procedures have been proposed for the halftone processing procedure, but there is a procedure called the error diffusion method that is representative of these. The error diffusion method is a procedure whereby errors in the gradation expression that occur by dots being formed or by dots not being formed on the pixel of interest are diffused and stored in unjudged pixels at the periphery of the pixel of interest, and for judging the dot on/off state for the unjudged pixels, a judgment is made for the dot on/off state so as to eliminate error diffused from the peripheral pixels. With the error diffusion method, because halftone processing is performed while eliminating the gradation expression errors that occur with peripheral pixels in this way, it is possible to suppress the errors that occur due to conversion and to convert image data with good precision. It is reasonable that with the error diffusion method, each time the error is diffused to peripheral pixels, reading and writing occurs for memory, so it takes time to do the error diffusion process, and therefore there is a tendency for it to take time to display the images.